1. Field of the Invention
The invention relates to an air cushion guide for sheet or web-formed material, in particular for printed paper sheets in a printing press, by which guided sheet or web-formed material is supported on a supporting air cushion above at least one guide body or guide member formed with nozzle openings through which air is blown between the guide body or member and the guided material.
Such air cushion guides have been described, for example, in the published German Patent Documents DE 44 27 448 A1 and DE 42 42 730 A1. In a most varied form and design, they are used, among other purposes, for transporting freshly printed and yet wet sheets of paper in a delivery system of offset printing presses, for example, in a contact-free manner from a printing unit to a delivery pile or, in sheet turning or reversing devices, for transporting sheets to sheet transfer drums or the like, between two impression cylinders. In this regard, a problem arises that, depending upon the printing job, quite different types of paper, sometimes printed on both sides thereof, have to be fed safely, i.e., without smearing. However, with an air cushion guide of fixed characteristics, which are determined by the number and form of the nozzles and by the amount of air blown through the nozzles, this cannot be assured in a like manner for all types of paper.
In general, one would assume that the risk of smearing would be all the less, the greater the height at which the sheet floats above the guide members. This is not quite true, however, because, in air cushion guides which operate on the principle of the hydrodynamic paradox, stability of guidance depends upon the height of the air cushion. Thicker air cushions are less stable, i.e., the restoring forces exerted by the air cushion on the guided sheets when changes in spacing occur are much less thereat than in floating guides where there is only a slight spacing between the guide member and the sheets and where, because of a high flow speed of the air flowing out of the nozzles, the guided sheet is guided quite stably, that is, with high restoring forces. The latter is the more optimal solution especially for thin, yielding papers, whereas, a too small spacing from the guide baffles is problematic for stiff, thick paper qualities. It would therefore be optimal if one could realize an air cushion guide which simultaneously combines both a large spacing of the guided sheet and great stability because of a high flow speed of the blown or blast air under the sheet. It is self-deceiving, however, to assume that this could be achieved with an air cushion operating in accordance with the aerodynamic paradox, by simply xe2x80x9copening up the blowerxe2x80x9d and thus lifting the sheet by blowing a greater amount of air into the air cushion. This becomes clear from the graph in FIG. 4, wherein the operating conditions of an air cushion guide according to the prior art are shown. Specifically, the curve c thereof shows the dependency of the speed c of the airflow blown through the nozzles into the air cushion, and the curve Q shows the volumetric flow Q, respectively, dependent upon the initial or supply pressure Pv of the chamber in the guide member from which the nozzles are supplied. If the pressure is increased, both variables vary approximately to the same extent or, in other words, approximately proportionally to one another. Conversely, the flotation height, as the curve h shows, remains virtually the same when pressure variations occur over a relatively wide range between 0.5 millibars (mbar) and 10 mbar.
Because it was consequently impossible to adjust the flotation height of the guided sheets to the various paper qualities by controlling the air compressor used for the air cushion guide, other courses were taken heretofore. For example, the hereinafore mentioned, published German Patent Document DE 42 42 730 A1 teaches disposing the air openings or nozzles in interchangeable replacement cassettes, i.e., matching the air cushion guide to the guide material is accomplished by replacing cassettes. This is unable to be effected during operation of the printing press, however, nor can it be automated.
In the published German Patent Document DE 42 09 167 A1, a sheet guiding device is described wherein the flotation height of the sheet in the middle portion thereof is increased by additional blower nozzles from which airstreams or flows are blown which strike the sheet surface perpendicularly and lift the sheet in the middle thereof by the impulse effect of these additional airstreams. Although this may possibly allow the flotation height or level to be set uniformly over the width of the sheet, it does not produce an overall change in the flotation height.
A combination of nozzles which operate in accordance with the hydrodynamic paradox, and blower or blast nozzles directed perpendicularly to the guided paper web so as to increase the flotation height of the guided web and make it more uniform have been described for weblike materials hereinbefore in German Patent 17 74 126. However, this reference discloses no possible way of adapting or matching the flotation height to various material qualities during operation of the device.
From German Patent 20 20 430, it has become known heretofore for somewhat airfoil-shaped air cushion guide members, for guiding weblike materials, to be switched over mechanically in such a manner that at least two stable zones are produced for the spacing between the guided web and the guide member. The characteristic of the guide member is varied so that it acts, on the one hand, as an air cushion nozzle and, on the other hand, as an airfoil nozzle, i.e., in accordance with the hydrodynamic paradox. In this regard, however, the greater spacing of the guided part, which results from the air cushion characteristic, is achieved at the cost of reduced stability of the air cushion produced by this type of nozzle.
It is accordingly an object of the invention of the instant application to provide an air cushion guide which, even during operation, can be adapted or matched to the various properties of the guide materials and, in fact, so that, in particular, the flotation or suspension height of the guided sheet and the guided web, respectively, is also able to be varied by relatively simple automation processes.
With the foregoing and other objects in view, there is provided, in accordance with one aspect of the invention, an air cushion guide for sheet or web-formed material, comprising at least one guide member formed with nozzle openings through which air is blown between the guide member and the guided material for supporting the guide material on a supporting air cushion located above the guide member, at least one of two variables consisting of volumetric air flow emerging from the nozzles and flow speed between the guide member and the guided material being adjustable independently of one another so that a proportionality between the two variables is neutralized or nullified.
In accordance with another feature of the invention, an active number of the nozzle openings covered by the guided material is variable.
In accordance with a further feature of the invention, the guide member has a plurality of groups of the nozzles, each group of the nozzles being supplied with blowing air and being cut off therefrom in a separately connectible and disconnectible manner, respectively.
In accordance with an added feature of the invention, the groups of the nozzles are connected to a common blown air generator via control valves.
In accordance with an additional feature of the invention, each of the groups of the nozzles is connected to a separate blown air generator.
In accordance with yet another feature of the invention, effective cross sections of at least one of the categories of individual ones of the nozzle openings, of individual groups of the nozzle openings, and of all of the nozzle openings are variable.
In accordance with yet a further feature of the invention, the air cushion guide includes electrically actuated movable blocking members for varying the cross sections of the nozzle openings.
In accordance with yet an added feature of the invention, the movable blocking members are selected from the groups consisting of flaps and slides.
In accordance with yet an additional feature of the invention, the nozzles have controllably deformable flaplike, yielding tongues.
In accordance with still another feature of the invention, the air cushion guide includes electrically actuatable adjusting gears for deforming the tongues.
In accordance with still a further feature of the invention, the tongues are formed as bimetal strips, and an airflow heater is provided.
In accordance with still an added feature of the invention, the tongues are deformable under the influence of a pressure difference developing at the nozzles.
In accordance with still an additional feature of the invention, the guide member includes at least two groups of the nozzles, the groups having different consumer characteristic curves, the groups of the nozzles being suppliable by blown air at pressures regulatable independently of one another.
In accordance with another feature of the invention, the sums of throttle areas of the nozzles of both of the groups differ from one another by a factor of at least two.
In accordance with a further feature of the invention, the groups of the nozzles, respectively, are connected to different types of blown air generators.
In accordance with an added feature of the invention, the types of blown air generators are selected from the group consisting of blowers, ejectors and axial fans.
In accordance with an additional feature of the invention, the air cushion guide includes an electronic control unit into which one of a nominal flotation height of the material guided by the air cushion, and of an extent of variation of the nominal flotation height is inputtable as a reference value, the control unit being operatable for ascertaining at least one of a set of controlled values for a variation in air volume flowing out of the nozzle openings, and a variation in flow speed between the guide member and the guide material.
In accordance with another aspect of the invention, there is provided in a delivery system of a sheet-fed offset printing press, an air cushion guide for printed sheet or web-formed material, comprising at least one guide member formed with nozzle openings through which air is blown between the guide member and the guided material for supporting the guide material on a supporting air cushion located above the guide member, at least one of two variables consisting of volumetric air flow emerging from the nozzles and flow speed between the guide member and the guided material being adjustable independently of one another so that a proportionality between the two variables is neutralized or nullified.
In accordance with a further aspect of the invention, there is provided in a region wherein one of a sheet transfer device and a sheet turning device is located between two impression cylinders of a sheet-fed offset printing press, an air cushion guide for printed sheet or web-formed material, comprising at least one guide member formed with nozzle openings through which air is blown between the guide member and the guided material for supporting the guide material on a supporting air cushion located above the guide member, at least one of two variables consisting of volumetric air flow emerging from the nozzles and flow speed between the guide member and the guided material being adjustable independently of one another so that a proportionality between the two variables is neutralized or nullified.
In accordance with an added aspect of the invention, there is provided a method for adjusting a flotation height of sheet or web material guided in an air cushion guide, which comprises supporting the guide material on a supporting air cushion via at least one guide member, and blowing air beneath the guide material via nozzles in the guide member, a quotient between volumetric air flow blown in through the nozzles, and flow speed of the air between the guide member and the guide material being varied.
In accordance with another mode, wherein the guide member includes at least two groups of nozzles with consumer characteristic curves for each of the groups differing from one another by a factor of at least two, the method includes varying a ratio to one another of the pressures of blowing air with which the groups of nozzles are supplied.
In accordance with a concomitant mode, the method of the invention includes varying effective cross sections of the nozzles or individual groups of the nozzles.
The invention is thus based upon the recognition that the flotation height for an air cushion guide operating in accordance with the hydrodynamic paradox, can be markedly varied only if the proportionality between the volumetric air flow emerging from the nozzles and the flow speed of the air between the guide member and the guide material is neutralized or balanced out. To achieve this, the volumetric air flow and/or the flow speed of the air are adjusted independently of one another, and thus the quotient between these two variables is changed.
By way of this provision, it is not only possible to adjust the flotation height of the guided part to various values while preserving the stability provided by the principle of the hydrodynamic paradox, but also, in addition, by targeted, feedback-free changes in the volumetric air flow and the kinetic energy of the supporting air cushion, the air cushion guide can also be adapted optimally to other factors which occur during operation in printing presses, such as the subject and degree of moisture absorption by the printed sheet, incident centrifugal forces, turbulence, airstreams of hot-air dryers, and so forth. Thus, the pressman is provided with an additional method of exerting influence upon the guide sheet and of optimizing the outcome of the printing process.
One option for independent adjustment of the aforementioned two variables is to vary the number of active nozzle openings covered by the guide material. This occurs, for example, when the guide body includes a plurality of groups of nozzles, and each group of nozzles is supplied with blowing air in a separately connectible and disconnectible manner. The groups of nozzles can be connected to a common blown air generator via control valves, or each group of nozzles can be connected to a separated blown air generator. By press or machine-controlled actuation of the control valves or activation of the blown air generators, the volumetric air flow under the guide material or sheet can thus be increased overall, without any change in the flow speed of the air. In this way, the flotation height of the guide material or sheet is increased without sacrifices of guidance stability.
Analogously, it is possible to vary the effective cross sections of individual nozzle openings, of individual groups of nozzle openings, or of all of the nozzle openings, for example, by electrically actuated flaps, slides, or the like.
Thus the nozzles may have flaplike, yielding tongues, which are deformable, for example, via electrically actuatable adjusting gears. If the tongues are suitably formed as bimetal strips, the cross section of the nozzle opening can then also be varied via the temperature of the airstream, or if the tongues are slightly resilient, this can be effected under the influence of the pressure difference that then develops at the nozzles.
In an especially advantageous exemplary embodiment of the invention, the guide members have at least two groups of nozzles, and the groups have different consumer characteristic curves or, in other words, different dependencies of the volumetric flow admitted through the nozzle openings, upon the supply pressure pv1 of air present at the nozzle openings. For example, if the consumer characteristic curves differ by at least a factor of two, then the quotient of the total volumetric flow W blowing into the air cushion, and the flow speed c of the supporting air effectively developing under the guide material, and thus the flotation height, can also be varied by controlling the ratio of the pressures pv1 and pv2 of the two nozzle groups. The degree of influence is naturally greater, the greater the difference between the consumer characteristic curves of the two groups of nozzles, so that it also then becomes expedient to operate the groups of nozzles by different types of blown air generators, such as gas blowers, ejectors or axial fans, which intrinsically make available different magnitudes of initial or supply pressures and volumetric flows.
The embodiment of the invention can be automated especially well, because control of the flotation height requires merely controlling independently of one another the rotary speeds of the blown air generators supplying the two groups of nozzles.
In the interest of producing the simplest possible automation, it is also expedient to provide an electronic control unit, to which the nominal flotation height of the material guided by the air cushion, or the extent of variation thereof, can be input as a reference value, the control unit thereby ascertaining controlled variables for the variation of the air volume flowing out of the nozzle openings and/or the variation of the flow speed between the guide member and the guide material. The ascertainment of the controlled variables can be accomplished based upon families of one or two-dimensional characteristic curves stored in memory beforehand.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an air cushion guide and method of adjusting a flotation height, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, wherein: